Liquid crystal displays (LCDs) are generally constructed of two glass panels with a thin layer of liquid crystal sealed between them, this assembly known in the art as a “stack”. Polarizing films are mounted to both sides of the stack. Transparent electrodes on one of the glass panels of the stack receive a voltage, with the resulting electric field being impressed on adjacent liquid crystal molecules of the liquid crystal material, causing the molecules to change their orientation. This change of orientation of the liquid crystal molecules occurs within a volume of the stack between the electrodes. Where the electrode is relatively large, as in a numeric watch display, a corresponding relatively large volume of liquid crystal material is affected. Where the electrode is tiny, as in pixels of a television screen or computer display, the affected volume of liquid crystal material for each electrode is correspondingly tiny. Because liquid crystal molecules are inherently birefringent, an ability to electrically adjust the liquid crystal molecule orientation at each pixel allows control over the amount of light that passes through the polarizing screens on each side of the stack for that pixel. As is well known in the art, the basis for LCD television and computer screens is an array of a multitude of tiny, transparent electrodes that form pixels, each with electrically adjustable light transmittance characteristics that are adjusted by varying a voltage level applied to respective electrodes. For a large computer display or television, the number of pixels in the LCD screen may run into the millions.
A wide variety of LCD designs are in existence. Referring to FIG. 1, designers can choose the rubbing direction at which the director, or molecular axis, of the liquid crystal molecules orient when at rest, i.e. without application of a voltage to a respective electrode, with respect to the first glass surface. By appropriate selection of the rubbing direction of the second glass surface, twist angle Φ can be controlled. Referring to FIG. 2, the cell gap, d, which is the space between the glass panels filled with liquid crystal material, and the pre-tilt angle Θ can also be controlled, where the pre-tilt is the angle between the liquid crystal director and the glass surface. In addition to selecting these cell parameters, designers also select orientation of the polarizers mounted to the outer surfaces of the cell, as well as any birefringent films placed between the liquid crystal cell and the polarizers.
As one example, a common LCD design is the twisted nematic (TN) configuration, in which the twist angle Φ is chosen to be 90°. A typical TN LCD might have a pre-tilt angle Θ of 8°, and a cell gap of 5 microns or so. Other designs include the super twisted nematic (STN) mode with twist angles between 180 and 270 degrees, the in-plane switching (IPS) and optically compensated birefringence (OCB) modes with twist angles of 0°, and the vertically aligned nematic (VAN) mode with pre-tilt angles of nearly 90°. Many other modes. have also been designed and developed. Each of these designs has its own particular strengths and weaknesses. Some designs have superior field-of-view characteristics, while others have superior switching response times, and others may have the lowest manufacturing costs.
Regardless of the cell or panel design, the ultimate performance of an LCD depends on manufacturing the cell with the correct values of the rubbing direction, twist angle, cell gap, and pre-tilt angle. A variety of techniques and instruments have been introduced for measuring some or all of these parameters. However, these techniques and instruments are often slow, taking something on the order of 20-30 seconds or so to measure a single small location on a liquid crystal cell. As a result, during manufacture, only 5 locations on a cell might be tested, these locations being in the center and generally in each corner region. Such measuring detects cell defects such as misalignments in the rubbing direction, non-uniformity of the cell thickness and other defects.
The instruments for measuring these parameters are critical tools for production and quality control of LCD cells or panels, as well as for research and development. The slow rate of current measuring devices and methods described above obviously limits throughput of cells during manufacture. Existing instruments for measuring these parameters also frequently require several of the parameters to be known a priori and might only provide accurate measurements across a limited range of values. For example, an existing measurement system might require that the rubbing direction of a cell be known in advance, and might only be able to measure pre-tilt angles in the range of 0 to 30 degrees. Another existing measurement system might be unable to differentiate between clockwise and counterclockwise twist sense in the liquid crystal molecules.
Applicant's invention is capable of simultaneously measuring the rubbing direction, twist angle, cell gap, and pre-tilt of any liquid crystal cell. The present invention also has significant advantages over the prior art. As described below, the prior art describes. techniques that only measure a subset of the desired parameters, or that only work for a particular mode of cell, or that require some of the desired parameters to be known in advance, or that require the LC cell to be rotated in order to complete the measurement. The present invention has none of these limitations.
The invention disclosed in U.S. Pat. No. 5,239,365 describes a technique for measuring the thickness of a twisted liquid crystal cell. However this technique requires a priori knowledge of the rubbing direction and twist angle. By aligning linear polarizers in the appropriate direction, this technique can determine the retardance of the cell from spectral transmittance measurements, and then can calculate the cell gap based on the known birefringence An of the liquid crystal material. This technique is lo unable to measure the twist angle or rubbing direction.
The invention disclosed in U.S. Pat. No. 5,532,823 improved on the prior art. By providing spectral transmittance measurements through crossed polarizers, and by allowing continuous rotation of the liquid crystal cell between the polarizers, an approximation method is used to determine twist angle, rubbing direction, and cell gap. This technique requires that the twist angle is less than 120°, and therefore cannot be used to measure STN mode cells.
A further improvement is described in U.S. Pat. No. 6,081,337. In this technique, the liquid crystal cell is not rotated. Instead, polarizers before and after the cell are rotated while spectral transmittance measurements are made. An algorithm is described whereby the appropriate rotation angles for the two polarizers are determined and the rubbing direction, twist angle, and cell gap can be determined.
The three techniques described above each determine the properties of a liquid crystal cell by illuminating the sample with linear polarization states and analyzing the linear polarization component of the light exiting the sample. However, the chiral structure of a twisted liquid crystal cell is such that a significant amount of additional information can be obtained by investigating circular and elliptical polarization states. U.S. Pat. No. 6,300,954 recognized the usefulness of examining the full polarization state (Stokes vector) of light exiting the liquid crystal cell. This technique, however, only launches linear polarization into the cell. The cell is rotated in order to find an orientation that causes a measured maximum or minimum in the transmitted beam. At a located orientation, cell gap and twist angle can be determined from the measured Stokes vector. However, even this recent prior art requires rotation of the liquid crystal cell for measurement, and does not measure rubbing direction.
In the present invention, it is shown that to quickly and accurately measure cell gap, twist angle, and rubbing direction of a liquid crystal cell, it is advantageous to measure the complete Mueller matrix of the cell at one or more wavelengths. It is well known that accurately measuring the Mueller matrix of a sample requires illuminating the sample with a variety of polarization states such as linear, elliptical, and circular, including left-handed and right handed rotations, and analyzing a similar variety of polarization states after they interact with the sample. If the measurements are performed properly, the Mueller matrix of the sample can be measured. The Mueller matrix contains within it all possible polarization-altering properties of the sample, including retarder properties, polarizer properties, and depolarization properties. Prior to the present invention, there have been only a few papers that describe the theoretical or measured Mueller matrices of a liquid crystal cell.
The description in “J. Opt. Soc. Am.” (Vol. 68, pages 1756-1767, 1979) teaches a way to mathematically derive the Mueller matrix for a twisted nematic liquid crystal cell. However, no further analysis is provided, and no experimental results are shown. In “Appl. Opt.” (Vol. 37, pages 937-945, 1998), the mathematical derivations of the Mueller matrix of twisted nematic liquid crystal cell are furthered by calculating the polarization eigenstates of the theoretical Mueller matrices. However, no measurements are presented, and the purpose of this work was to find the polarization eigenstates of twisted nematic liquid crystal cells so that the devices could be used to achieve phase-only modulation for use, presumably, in optical correlation or other optical computing applications. That theoretical work actually followed the experimental measurements described in “Opt. Lett.” (Vol. 18, pages 1567-1569, 1993), which is the only reference we are aware of that shows the measured polarization eigenstates of a twisted nematic liquid crystal device. These measurements were made at a single wavelength, and were made as a function of applied voltage to the liquid crystal. The purpose of this work was to find these eigenstates so that the device could be used as a phase-only modulator for optical correlation applications. Finally, in “Meas. Sci. Technol.” (Vol. 12, pages 1938-1948, 2001), we find the only other set of Mueller matrix measurements on liquid crystal cells that we are aware of. In that article, investigations of crystal asymmetries and switching response times of ferroelectric liquid crystal cells are made using Mueller matrix measurements.
To summarize our survey of the prior are, it is seen that the prior art can be separated into two categories: patents that describe methods and apparatuses for measuring the physical properties of liquid crystal cells, and academic research papers that theoretically model or experimentally measure the Mueller matrix of liquid crystal cells. The patented techniques have been evolving from simple systems that were limited in their capability toward more complex systems that can measure more polarization properties, and thus, more parameters of the liquid crystal cell. However, no patented technique has yet advocated the level of, system complexity required for complete polarization characterization, that is, the full Mueller matrix measurement. The academic research papers have either derived what the Mueller matrix for liquid crystals should be, or have measured the Mueller matrix of liquid crystal cells with analysis for various research purposes. The purpose of these papers has always been to investigate the optical properties of LC cells. These papers have not considered inverting the problem, that is, using the measured optical properties to go back and determine the physical properties of the cell. None of these papers have presented experimental measurements or theoretical analysis that advocates the use of full Mueller matrix measurements for simultaneously and uniquely determining the rubbing direction, twist angle, cell gap, and pre-tilt of liquid crystal cells.
It is, therefore, one object of the present invention to provide a measurement method of the Mueller matrix of liquid crystal cells wherein one or more of the parameters comprising cell gap, twist angle, and rubbing direction can be determined exactly even if their values are previously unknown.
Another object of the present invention is to provide a measuring apparatus for measuring the optical properties of liquid crystal cells, even if the cell gap, twist angle and rubbing direction of the cell are previously unknown, and which does not require the liquid crystal cell to be rotated during the measurement.
Another object of the present invention is a method of manufacturing liquid crystal devices with desired values of cell gap, twist angle, and rubbing direction.
Another object of the present invention is to provide a measurement method of the Mueller matrix of liquid crystal cells where the pre-tilt of the liquid crystal directors is determined by varying the incident angle of the measurement beam on the liquid crystal device, and where the pre-tilt angle can be any value from 0 to 90 degrees.
Another object of the present invention is a measuring apparatus for measuring the pre-tilt of the liquid crystal directors of a liquid crystal cell, where the pre-tilt angle can be any angle from 0 to 90 degrees.
Another object of the present invention is a method of manufacturing liquid crystal devices with a desired value of the pre-tilt angle.
Further objects of the invention will become apparent upon a reading of the following appended specifiction